1. Field of the Invention
The invention relates to a process for depositing metal coatings on polyimide surfaces, and to a process for producing an electrical circuit base, which may be made by using said process.
2. Description of the Related Art
Polyimide is used in the electronics industry as a substrate material for producing printed circuit boards, hybrid circuits, semi-conductor carriers (chip carriers, multi-chip modules) and other components. This material has advantages compared to previous material such for example as epoxy resins.
For example its thermal resistance is higher, so that the longitudinal expansion of the material under thermal stress is less than with previous materials. Polyimide substrates also have better electrical insulating values.
For use as a substrate material in semi-conductor carriers, semi-finished products of the polyimide can be applied to an appropriate carrier by a spin-coating process in a thin layer and subsequently converted to polyimide. It is possible in a simple and reproducible way to etch micro-fine holes in the coatings thus formed, these holes serving to connect a plurality of metallization planes.
Polyimide films lined with copper films are usually used in the production of polyimide laminates, for example for the manufacture of printed circuit boards. The conductor tracks are usually produced from the copper coatings by etching processes. For this purpose process techniques are used which have been described at many points in the literature (for example in Handbuch der Leiterplattentechnik,ed.:. G. Herrmann, Vol. 2, Eugen C. Leuze Verlag, Saulgau 1991). Such processes are basically suitable for the manufacture of printed circuit boards, yet the width of the finest conductor tracks which may be reproducibly manufactured by such techniques lies in a range of roughly 75-100 .mu.m.
The copper films are connected in a previously known way by adhesion with the polyimide surfaces. However, the adhesive coating softens under thermal stress, for example when the printed circuit boards are soldered, and is also insufficiently resistant to the chemical baths used for metallizing holes in polyimide laminates.
In order to avoid the adhesive coatings, there was developed the "cast-on" technique well known to the person skilled in the art, for manufacturing adhesive-free polyimide laminates, in which liquid polyamide acid solution is poured on to a copper film before dehydration and cyclisation to form the polyimide. After formation of the polyimide on the copper film, there results in this way a securely adhering polymer/metal bond. This process also has the disadvantage that only relatively thick copper films, for example 17 .mu.m thick, can be used. In addition, this material is extremely expensive, for example the commercial product Espanex.RTM. of Nippon Steel Chemical Co. Ltd., Tokyo.
Polyimide laminates coated with thin copper films were in fact manufactured. However the outlay for manufacturing these laminates is extraordinarily high, so that the material costs are also considerable. Handling of the laminates coated with thin copper films is also problematic, as the copper films are extremely sensitive to mechanical influences. In the case of the "cast-on" technique, such thin copper films cannot be used, as the laminate would be severely distorted, among other things, during manufacture.
Finer conductor tracks may however still be produced, when no laminates provided with copper films on the surface are used as initial materials. In this case the conductor tracks are formed directly on the surfaces by metal coating. Adhesive-free polyimide substrates produced by sputtering or metal evaporation or by means of chemical methods have not for some time become established in the manufacture of printed circuit boards. For example, it is necessary in order to produce a sufficient adhesion of the metal coatings on polyimide first to apply a thin chromium layer, upon which the copper coatings are then deposited, for example pre-drilled material in a grid pattern of the company Sheldahl. However, chromium gives rise to problems in etching, so for example an additional etching process is necessary, for which reason the use of chromium coatings is avoided where possible. From a technical standpoint it would therefore be extremely desirable to manufacture circuit bases from uncoated polyimide material. It has not in practice yet been possible to realise this technically. The following are the reasons for this:
Particularly when chemical metal coating methods are used, the disadvantage emerges of the greater water capacity of polyimide compared to other polymers. It has for example become apparent that the adhesion of copper coatings deposited in an electroless and electrolytic manner on the entire surface of polyimide films on the polyimide surface, is considerably reduced under thermal stress, for example during soldering, or is in fact entirely removed. In order to avoid the occurrence of bubble-shaped raised portions in thermal treatment, it is generally proposed to anneal the coated polyimide films after coating. The annealing treatment alone however is insufficient to prevent the occurrence of bubbles, when polyimide films provided on the entire surface with copper coatings on both sides are exposed to thermal treatment. In this case the enclosed moisture originating from the chemical metallizing process can no longer escape during the annealing treatment, so that water emerges explosively from the polyimide film during thermal treatment and detaches the copper coating from the polyimide surface.
For the abovenamed reasons, other processes for securely adhesive metallizing of polyimide substrates have been developed. Due to the requirements to obtain sufficiently secure adhesion of the deposited metal coatings both during and after thermal stress on the substrates, vacuum processes for metallizing have been used. Metal coating by the decomposition of volatile metal compounds by means of glow discharge represents a superior procedure in this respect. In DE 35 10 982 A1 there is disclosed such a process for manufacturing electrically conductive structures on non-conductors, for example polyimide films by the deposition of metallic films on the non-conducting surfaces by decomposition of organometallic compounds in a glow discharge zone. The deposited metal films preferably serve as catalytically active nucleus layers for the subsequent electroless metallization of the surfaces.
There is also described in DE 38 06 587 A1 a process for manufacturing metallic structures on polyimide by means of the decomposition of organometallic compounds in a glow discharge.
Further metallizing processes by means of glow discharge and organometallic compounds are disclosed in the documents DE 37 16 235 A1, DE 37 44 062 A and DE 38 28 211 C2.
In order to reduce the purest possible metal coatings by the glow discharge method, i.e. metal coatings without any appreciable admixtures of carbon and oxygen, it is proposed to heat the substrate up to the highest possible temperature during the metal deposition. The carbon-containing admixtures originate from the organic components of the volatile organometallic compounds usually used in deposition. The influence of the substrate temperature on the carbon content of the metal coatings deposited by the glow discharge method is for example disclosed in E. Feurer and H. Suhr, "Thin palladium films prepared by metal-organic plasma-enhanced chemical vapour deposition", Thin Solid Films 157 (1988),pages 81, 84. According to this, the carbon content reduces as the substrate temperature rises. It can also be seen from this document that the palladium coatings contain scarcely 30% by weight of carbon at ambient temperature.
If the metal coating contains an excessive carbon content due to insufficient decomposition of the metal compounds in the glow discharge, or due to insufficient desorption of the only partially decomposed ligand compounds from the substrate surfaces, the conductivity of the deposited metal coatings is relatively low and in addition its catalytic activity is insufficient to start an electroless metal deposition on the surfaces, so that metal coatings slightly contaminated with oxides are formed in an electroless manner, or deposition is totally absent.
Heating of the substrate is undesirable for many applications. Therefore attempts were made to reduce the carbon content of the metal coatings in another way. For example, in the above named publication by E. Feurer and H. Suhr it is proposed to provide post-treatment of the deposited metal coatings with an oxygen plasma. According to experiments by these authors, a further alternative resides in replacing the argon, which is conventionally used as a carrier gas in metal coating, totally by oxygen. Thus instead of palladium, palladium oxide is deposited, which can again be reduced to form metallic palladium by means of a subsequent treatment in a hydrogen plasma. This procedure is however extremely complicated and expensive. Moreover, the organometallic compounds already decompose when the oxygen is passed through the supply container containing these compounds, so that only a small proportion reaches the glow discharge zone.
It is in fact possible to produce secure adhering metal coatings on polyimide surfaces by the metal deposition process by means of a glow discharge process, if further metal coatings from electroless and electrolytic metallising baths are deposited on this metal coating. It has however become apparent that sufficiently secure adhesion of the deposited metal coatings on the polyimide surface does not occur if the coated substrate comes into contact with aqueous-alkaline solutions after the necessary annealing treatment and before or during the test for determining the adhesion. Considered as aqueous- alkaline solutions for example are the developer and strip solutions in the structuring processes with photoresists, in order to form conductor structures from the metal coatings deposited on the entire surface. Subsequent annealing treatment cannot reverse the reduction in adhesion caused by contact with the aqueous-alkaline solution.
Thus the problem underlying the present invention is to avoid the disadvantages of prior art and to find a process by means of which extremely pure metal coatings may be deposited in an electroless manner on polyimide surfaces, in order to produce micro-fine conductor track structures, the adhesion of the metal coatings on the surfaces not being impaired by contact after deposition with aqueous-alkaline solutions, and a process by means of which also micro-fine conductor track structures securely adhering to the polyimide surfaces can be produced in order to manufacture electrical circuit bases with polyimide surfaces.